Barium sulphate is used to a great extent for example in the dye, paint and lacquer, plastic, fibre, paper and rubber industry. It is distinguished in that as a totally inert filler it does not involve any reaction with other chemicals used in those branches of industry. In comparison with other solids such as calcium carbonate barium sulphate is distinguished by its very high chemical resistance. In particular the low Mohs hardness is advantageous, in comparison with silicon dioxide and aluminium dioxide.
In general terms barium sulphate is produced by the reaction of a barium salt such as that of the sulphide, chloride or nitrate with sulphuric acid or a salt thereof such as sodium sulphate in an aqueous solution. That reaction generally gives barium sulphate which usually has a primary particle size of 0.01 to 20 μm. The barium sulphate produced in that way is filtered off, washed with water, dried and deagglomerated for various purposes of use.
Optionally the barium sulphate particles are provided with additives prior to or after drying. The deagglomeration operation is usually performed by dry crushing for example using impact grinding mills. If deagglomeration of the barium sulphate particles in accordance with the state of the art is implemented, those barium sulphate particles can be well dispersed in organic and aqueous systems. A disadvantage of the barium sulphate particles produced in the state of the art however is the marked worsening in dispersibility in organic and aqueous systems due to storage.
DE-A1 33 47 191 discloses a process for the production of a surface-treated barium sulphate. That process involves using an aqueous BaSO4 suspension containing an excess of Ba ions. 0.1 to 30% by weight of an aqueous alkali silicate solution is added to that suspension so that initially barium sulphate precipitates. A mineral acid is then added and a pH-value of less than or equal to 7 is set for decomposition of the barium sulphate to give water-bearing silicon dioxide. The described process steps are preferably effected at temperatures of at least 40° C. The barium sulphate treated in that way however still does not have satisfactory dispersion properties, particularly if the barium sulphate has been stored over a prolonged period of time.
DE-A1 44 31 735 discloses a process for the production of an inorganically treated filler for polymers from BaSO4, wherein firstly H3PO4 or a water-soluble phosphate compound is added to an aqueous BaSO4 suspension. The process then involves the successive addition to the suspension of Na2SiO3 at a pH-value of less than 7.5 and an aqueous aluminium compound at a pH-value of greater than 4.5. The described process steps are performed at temperatures of 65 to 75° C.
Summarizing, the barium sulphates known in the state of art show remarkable worsening of dispersibility after storage.
Therefore the object of the invention is to provide a barium sulphate which even after prolonged storage has an improved dispersibility in organic and aqueous systems. SUMMARY OF INVENTION
It was now surprisingly found by the inventors that such a barium sulphate can be produced by means of the process having the features recited in the main claim.
Thus the present invention concerns a process for the production of a barium sulphate, in which:
a) an aqueous suspension of barium sulphate is set in the presence of an excess of sulphate ions to a pH-value of 9 to 12;
b) the resulting suspension of barium sulphate is mixed at a pH-value of 9 to 12 with at least one aqueous solution of a water-soluble metal compound or a mixture thereof;
c) the resulting suspension is set to a pH-value of 3 to 9 by the addition of at least one acid or acid mixture to precipitate a metal-oxygen compound of the water-soluble metal compound on the surface of the barium sulphate particles; and
d) the resulting barium sulphate particles are filtered off.